1. Field of the Invention
The present invention relates to a fabrication method of burnable absorber nuclear fuel pellets and burnable absorber nuclear fuel pellets fabricated by the same.
2. Description of the Related Art
Uranium dioxide (UO2) sintered pellets are most widely used nuclear fuels, which contains 1 to 5 wt. % of U235. As U235 disintegrates due to neutrons, nuclear fissile energy is generated. It is economically more beneficial, if the operation cycle of the nuclear reactor core is prolonged, thus increasing the operability thereof. It is thus beneficial to load nuclear fissionable material in the reactor core as much as possible to increase the operation cycle of the nuclear reactor core. However, too much nuclear fissionable material will cause too high reactivity in the initial stage of the cycle, thus affecting safety of the nuclear reactor core. Accordingly, for the purpose of regulating neutrons, burnable absorber sintered pellets containing burnable absorber material with high absorption cross-section such as gadolinium (Gd) or erbium (Er) can be utilized. These rods of burnable absorber sintered pellets are sometimes called “poison rods” due to poisonous property thereof.
Attempt has been made in 1960 by Combustion Engineering Inc. (USA) to fabricate burnable absorber sintered pellets (or “poison rods”) by uniformly adding boron (B) compounds including B4C as burnable absorber materials. However, there were drawbacks such as formation of B2O3 with low melting point and low boiling point by the reaction between excess oxygen of UO2 powder with boron during sintering, which easily volatilizes, and increased internal pressure due to generation of helium during the reaction of 10B+1n→11B(excited state)→4He+7Li.
Meanwhile, the researchers of Irradiation Engineering Inc. observed oxidation behavior of UO2—B4C blend at a temperature range of 325° C.-1600° C. The observation confirmed that the oxidation reaction with the excess oxygen within UO2 of B4C occurs across the entire range of the test temperature, and that formed B2O3 rarely volatilized under 1200° C. It was also reported that the remaining boron compounds after volatilization formed UB4 phase at a temperature between 1250° C. and 1300° C., which were thus present in UO2. However, to address the relatively high volatility during sintering, it was necessary to add excess amount of excess boron compounds in an initial stage in consideration of the volatilization amount to ensure that the boron remains in the sintered pellets in the amount that is sufficient to act as the burnable absorber pellets. However, the excessive amount of volatilized boron caused shortcoming of severe degradation of density of sintered pellets. Accordingly, the sintered density did not exceed 90% TD (theoretical density) even when the sintering was done at the temperature of 1600° C.
The Kraftwerk Union Aktiengesellschaft (Germany, U.S. Pat. No. 4,774,051) later reported production of sintered compacts with uniformly dispersed boron therein, by mixing 2-100 μm UBx (X=2, 4, 12) and B4C powder with 15 μm UO2 powder, respectively, and sintering the respective mixtures under reducing and slightly oxidizing atmospheres. Although the research reports that the sintered compacts with sintered density exceeding 95% TD were produced by synthesizing UBx (X=2, 4, 12) powder with UO2 powder in advance, and then sintering the same at 1700° C., the temperature similar to the conventionally applied temperature of producing sintered compacts by utilizing reducing atmosphere such as hydrogen gas, there was a difficulty in implementing the process in the conventional sintered pellet producing process because it was necessary to synthesize UBx powder in advance.
It was also reported that sintered compacts with homogenously-dispersed boron at a sintered density of 95% TD (theoretical density) can be fabricated when sintering is performed at low temperature of 1150° C. under slightly oxidizing atmosphere using CO2 gas, and this is considered to be attributable to the following two typical phenomena that occur when UO2 is sintered under slightly oxidizing atmosphere.
Certain slightly oxidizing atmosphere sintering provides the effect of capturing B4C within grains due to fast grain boundary migration (abnormal grain growth) in AUC-UO2. Further, because of higher oxygen partial pressure of CO2 gas atmosphere than reducing gas atmosphere, the oxygen/metal (O/U) ratio of UO2 increases, and as U diffusion coefficient increases, it is possible to densify UO2 compacts even at temperature as low as 1150° C. which does not have sufficient volatilization of B2O3. Accordingly, it is possible to reduce open porosity and boron volatilization can be effectively suppressed because passages for leaking of volatilized B2O3 out of the sintered compacts are removed.
However, because the above phenomena are only observed in UO2 powder made by AUC (wet) process, it is hardly implementable to the other UO2 powders, such as DC-, IDR-, ADU-route UO2 powders. Further, in terms of fabrication, because CO2 gas is used as the sintering atmosphere, the method is hardly compatible with the conventional nuclear fuel sintered compact fabricating process which utilizes reducing atmosphere sintering, and the capturing of CO2 gas with low diffusivity within the porosity of the sintered pellets causes much more swelling of nuclear fuel to aggravate during the irradiation in nuclear reactor. The gases such as CO and CO2 do not normally escape from closed pores in the sintered UO2 pellet because of lower solubility than that of H2 and O2.
Due to the problems mentioned above, it has been never reported that the burnable absorber sintered pellets with uniformly-dispersed boron therein are used in the commercial light-water reactor. That is, the currently-available light-water reactor uses sintered pellets in the form in which boron is coated to surfaces thereof. Studies have been conducted to fabricate integrated burnable poison sintered pellets by uniformly adding SiB4 to UO2, but no attempt of commercialization has been made so far.